1. Field of the Invention
The present invention relates to a developing device for a copying machine or a printer which employs an electrophotography.
2. Description of the Related Art
The inventors of the present invention know a developing device which includes a developing tank 12 disposed adjacent to a photoconductor 11 and a toner supply tank 14 for supplying magnetic toner 13 to the developing tank 12, a separating plate 15 disposed between the developing tank 12 and the toner supply tank 14, and a toner supply port 16 formed below the separating plate 15 for connecting the developing tank 12 and the toner supply tank 14 to each other, as shown in FIG. 1.
The developing tank 12 is composed of a developing roller 18 rotatably supported by a developing device body 17 and a doctor 20 for restricting the amount of developers (the toner and a carrier) 19 which are attracted to a surface of the developing roller 18. As shown in FIG. 2, the developing roller 18 includes a sleeve 21 which rotates in the reverse direction to the direction of rotation of the photoconductor 11 and a magnetic body 22 disposed on the inner surface of the sleeve 21, the magnetic body 22 being arranged to rotate in the reverse direction to the direction cf rotation of the sleeve 21.
The toner supply tank 14 includes a toner stirring roller 23 rotatably disposed so as to stir and supply a magnetic toner 13 to the developing tank 12 through the toner supply port 16.
As a result of the above-described structure, the developers (the toner and the carrier) 19 enclosed in the developing tank 12 are moved by a rotation of the sleeve 21 and that of the magnetic body 22 with the amount of the developers on the surface of the sleeve 21 restricted by the doctor 20. The developers 19 are further moved until to reach the photoconductor 11 so that an image is developed.
Furthermore, two-component developing devices which use the magnetic toner are disclosed in Japanese patent application Laying Open No. 60-75854, No. 62-5279, No. 63-6572, No. 63-83753 and No. 63-129366.
However, in any one of the above-mentioned developing devices, the developers including the magnetic toners and the carriers encounter a problem in that the magnetic toners supplied from the toner supply tank and the developers cannot be satisfactorily stirred. Furthermore, the pressure and the fluidity of the developers in a stirring chamber or the circulating passage are changed due to the deterioration in the developers and to the humidity as well as the temperature. As a result, another problem arises in that the toner density in the developers becomes too unstable.
In general, the development is performed by each of the printers in accordance with the inverse developing method. In the case where a solid black pattern (the ratio of a black portion and a white portion is 100%) is printed after the originals 24 and 25 respectively shown in FIGS. 3 and 4 were successively printed in 200 paper sheets, then the documents 26 and 27 shown in FIGS. 5 and 6 respectively are obtained. As a result, partial solid white portions 28, 29 are formed in the portions in which there is no image, while the image portions are free from the solid white portions 28, 29.
A comparison is made between the amount of the charge of the magnetic toners on the sleeve of the developing roller at the positions which correspond to the non-image portions in which the solid-white portions 28, 29 are formed and that at the positions which correspond to the image portions in which no solid-white portions 28, 29 are formed. As a result, it is found that the amount of the charge of the magnetic toners at the positions corresponding to the non-image portions are lower than the other portions. It might be considered that the reason for this lies in a fact that an electric influence on the image portions and that on the non-image portions are different from each other at a time when the portions contact with the photoconductor 11.
Therefore, the developers on the sleeve in the non-image portions are always in contact with portions of the photoconductor 11 in which the electrical potential is high (it is usually 400 V or higher). As a result, the charge of the developers is discharged, causing the amount of charge of the magnetic toners to be reduced. It leads to a fact that the amount of the magnetic toners to be transferred to the photoconductor 11 is reduced. Therefore, the density of the image in the solid-black portions is undesirably lowered.
Furthermore, any one of the above-described developing devices encounters a problem in that the developers are, by falls or vibrations, moved to either side in the axial direction of the developing roller during the transportation of the developing tank from a manufacturing plant to a user when the developing tank is filled with the developers at the manufacturing plant. Therefore, when the magnetic toners introduced into the toner supply tank are supplied to the developing tank, the magnetic toner density becomes different in the axial direction, and as a result the image density differs in the axial direction.